The invention relates generally to the apparatus and method for noninvasively producing sphygmometric information by analysis of an oscillometric waveform.
The prior art is replete with devices for measuring the blood pressure of a living subject. The simplest and oldest device is a pressurizable cuff used in combination with a mercury manometer which reads the pressure in the cuff and a stethoscope which is used to listen to Korotkoff sounds. More complicated methods and apparatus based on the same principles of listening to the Korotkoff sounds replace the mercury manometer with a mechanical or electromechanical pressure gauge and utilize microphonic detection of the Korotkoff sounds which are amplified electrically. In another advanced method of measuring blood pressure, the distance of a blood pressure cuff to the wall of an artery is actually determined by measuring Doppler shifts of sound waves reflected by the artery. The distance to the artery varies as a function of pressure within the somewhat pliable walls of the artery. Certain multicuff techniques have also been employed and of course, intrusive devices have also been inserted directly into blood vessels to make direct blood pressure measurements.
The methods based on listening to Korotkoff sounds are inherently somewhat inaccurate for measuring diastolic pressure since the human or electromechanical listening must distinguish when a sound fades out. Further, the Korotkoff technique does not work well on newborn patients, obese patients and those patients suffering from shock or hypertension. The method based on doppler shifts, in addition to requiring expensive apparatus, is sensitive to the positioning of the measuring apparatus relative to the artery and requires a coupling gel. The intrusive methods are somewhat painful and require the attention of a trained medical technician. The multicuff techniques suffer from the requirement for multiple nonstandard cuffs and are somewhat prone to artifacts.
In addition to the aforementioned techniques, oscillometric techniques have been developed in which a pressure transducer associated with an inflatable pressure cuff provides an oscillatory signal representative of the pulsatile pressure fluctuation sensed by the pressure cuff as a function of volumetric changes in the embraced blood vessel. One type of oscillometric technique disclosed in U.S. Application Ser. No. 578,047, filed May 15, 1975 and now U.S. Pat. No. 4,009,709 by Link, et al. for Apparatus and Process for Determining Systolic Pressure, and incorporated herein by reference, relies on the amplitude of the oscillatory pulsations to provide an indication of systolic and/or diastolic pressure, however these techniques may be improved upon by the present invention particularly in the presence of various artifacts.
Recently a technique of waveform analysis of the oscillatory signal has been developed which provides superior results in identifying a patient's diastolic blood pressure. This technique is described in greater detail in U.S. Pat. No. 3,903,872 issued Sept. 9, 1975 to W. T. Link for Apparatus and Process for Producing Sphygmometric Information, and is incorporated herein by reference. In that technique, a pulsating signal representative of the sum of the applied pressure of the cuff and the sensed pulsatile pressure of the embraced blood vessel is differentiated. The resulting time derivative signal is evaluated at a specific time in a pulse (on the blood pressure waveform), and the time derivative is seen to go through a (negative) maximum value when the applied cuff pressure is approximately equal to the undistorted arterial pressure at that specific time within the blood pressure waveform of the time derivative maximum proximate to diastole in each heart beat was found to correspond with the diastolic pressure of the blood vessel.
While the technique disclosed in the aforementioned patent provides a powerful and significant advance in the art of sphygmomanometry, it requies relatively expensive apparatus to interpret the waveform and remains somewhat prone to interference from certain artifacts, though to a lesser degree than most earlier mentioned techniques.
Accordingly, it is a principle object of the present invention to provide an apparatus and method for relatively inexpensively and accurately analyzing oscillometric waveforms to provide an indication of the physical condition, and particularly the diastolic and/or systolic blood pressures of a living test subject.
An apparatus and a process for satisfying the foregoing and other objects is disclosed into the following.